The present invention is related to optical fibers and their manufacturing methods and, in particular, to optical fiber subassemblies in which a plurality of separate optical fibers are joined together.
A standard optical fiber has a relatively simple structure with a central core and a surrounding cladding. The core has one index of refraction and the cladding has a different index of refraction (typically less than the core index of refraction). The core and cladding have constant diameters along the length of the fiber. Depending upon the diameter of the core, the optical fiber can be a single mode fiber or a multimode fiber.
Variations from the standard optical fiber include fibers with multiple cores in which two or three parallel cores of constant diameters are enclosed by the surrounding cladding. As in the case of standard optical fibers, such multiple core fibers are manufactured with preforms and drawing techniques. Examples of such fibers include the polarization-maintaining optical fiber with its characteristic "mouse ears" cross-section. Another variation from the standard optical fiber is the thermally expanded core optical fiber in which the single core of the fiber is gradually enlarged in a taper. Such fibers are often used as light receptors because of the larger cross-sectional area of the core.
Recently optical devices have been developed in which two or more optical fibers are held in a sleeve. Light is transmitted (and received) through the end facets of the optical fibers. Such optical devices include WDM (Wavelength Division Multiplexing) couplers, add/drop multiplexers, tunable filters, optical switches, and so forth. See, for example, Nonetheless, it remains desirable to lower the manufacturing costs and reliability of these optical devices. For some purposes, the thermally expanded core optical fibers may be used since the larger cores increase manufacturing tolerances. However, such fibers are relatively expensive.
Hence there is a continued need for improvements in the manufacture of fiberoptic devices. The present invention provides for such an improvement for many fiberoptic devices.